locale: fix incomplete fallback bundles

[BAMF0]

Fix an error where only the common strings would be used when falling back to embedded locales. This lead to an issue where the raw Fluent message keys were returned rather than actual error messages, such as wc-error-failed-to-print-result instead of "failed to print result".

Additionally, preserve partial resources instead of discarding everything when not all messages could be parsed.

Fixes #11854

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will not alter performance

✅ 331 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing BAMF0:bamf0/11854-fix-incomplete-locale-bundles (d22a755) with main (2a92fcc)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

could you please add a test to make sure we don't regress in the future?
thanks

[BAMF0]

Ah yes of course! Good catch

[BAMF0]

There we go. It was a bit tricky to reproduce outside of the Ubuntu package build pipeline, but I managed to figure it out. I have updated my issue with the reproduction steps as well for future reference.